Circuit substrate and branch circuit

ABSTRACT

A circuit substrate includes a substrate body, an input signal line conductor with which the substrate body is provided and included in an input path, a first mounting portion provided on a main surface of the substrate body, included in a first output path and on which a high-pass filter including a lumped-parameter element is mounted, at least one first output signal line conductor with which the substrate body is provided and included in the first output path, and a second mounting portion provided on the main surface of the substrate body, included in a second output path and on which a low-pass filter including a lumped-parameter element is mounted. The first output signal line conductor provided farthest upstream in a signal propagation direction is connected to the input signal line conductor via a first lumped-parameter element mounted on the main surface of the substrate body.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit substrate and morespecifically relates to a circuit substrate preferably for use in abranch circuit including a high pass filter and a low pass filter. Inaddition, the present invention relates to a branch circuit includingthe circuit substrate.

2. Description of the Related Art

As an example of an invention relating to a circuit substrate of therelated art, the splitter described in Japanese Unexamined PatentApplication Publication No. 2005-323064 is known. FIG. 19 illustrates asplitter 100 described in Japanese Unexamined Patent ApplicationPublication No. 2005-323064.

The splitter 100 includes first to third ports P1 to P3 and a high passfilter hpf and a low pass filter lpf. The first port P1 is an inputport, and the second port P2 and the third port P3 are output ports. Asignal path connected to the first port P1 branches and is connected tothe second port P2 and the third port P3. In addition, the high passfilter hpf is provided between the first port P1 and the second port P2,and the low pass filter lpf is provided between the first port P1 andthe third port P3.

In the splitter 100 described in Japanese Unexamined Patent ApplicationPublication No. 2005-323064, the high pass filter hpf and the low passfilter lpf preferably include conductor layers provided in a circuitsubstrate of the splitter 100 and chip components provided on a surfaceof the circuit substrate. The port P1 and the high pass filter hpf areconnected to each other by a signal line composed of a conductor layerprovided in the circuit substrate. Since an unwanted inductor componentis generated in such a signal line, impedance matching between the firstport P1 and the second port P2 is disrupted. As a result, reflection ofa signal occurs between the first port P1 and the second port P2 andloss occurs in the splitter 100.

SUMMARY OF THE INVENTION

Accordingly, preferred embodiments of the present invention provide acircuit substrate and a branch circuit that significantly reduce orprevent generation of loss.

A circuit substrate according to a preferred embodiment of the presentinvention preferably is configured to be used in a branch circuit thatincludes an input path, a first output path including a high pass filterand connected to the input path, and a second output path including alow pass filter and connected to the input path. The circuit substrateincludes a substrate body; an input signal line conductor with which thesubstrate body is provided and which is included in the input path; afirst mounting portion provided on a main surface of the substrate body,included in the first output path and on which the high pass filterincluding a lumped-parameter element is mounted; at least one firstoutput signal line conductor with which the substrate body is providedand which is included in the first output path; and a second mountingportion provided on the main surface of the substrate body, included inthe second output path and on which the low pass filter including alumped-parameter element is mounted. The first output signal lineconductor provided farthest upstream in a propagation direction of asignal is connected to the input signal line conductor via a firstlumped-parameter element mounted on the main surface of the substratebody.

A branch circuit according to another preferred embodiment of thepresent invention includes the circuit substrate; the high pass filtermounted on the first mounting portion; the low pass filter mounted onthe second mounting portion; and a lumped-parameter element connectingthe first output signal line conductor provided farthest upstream in apropagation direction of a signal and the input signal line conductor toeach other, and mounted on the main surface of the substrate body.

According to various preferred embodiments of the present invention,generation of loss is significantly reduced or prevented.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit diagram of a branch circuit.

FIG. 2 is a plan view of the branch circuit from above.

FIG. 3 is a plan view of a circuit substrate from above.

FIG. 4 is a plan view of the circuit substrate from below.

FIG. 5 is a plan view of a branch circuit according to a modificationfrom above.

FIG. 6 is a graph illustrating a transmission characteristic |S21| in afirst sample.

FIG. 7 is a graph illustrating a transmission characteristic |S21| in asecond sample.

FIG. 8 is an equivalent circuit diagram of a first model correspondingto the first sample.

FIG. 9 is an equivalent circuit diagram of a second model correspondingto the second sample.

FIG. 10 is a graph illustrating a transmission characteristic |S21| ofthe first model and the second model.

FIG. 11 is an equivalent circuit diagram of a model used in a computersimulation.

FIG. 12 is a graph illustrating a reflection characteristic |S33| in athird model.

FIG. 13 is a graph illustrating a reflection characteristic |S33| in afourth model.

FIG. 14 is a graph illustrating a reflection characteristic |S33| in afifth model.

FIG. 15 is an equivalent circuit diagram of a branch circuit.

FIG. 16 is a plan view of the branch circuit from above.

FIG. 17 is a plan view of a circuit substrate from above.

FIG. 18 is a plan view of the circuit substrate from below.

FIG. 19 illustrates a splitter described in Japanese Unexamined PatentApplication Publication No. 2005-323064.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, circuit substrates and branch circuits according to preferredembodiments of the present invention will be described while referringto the drawings. FIG. 1 is an equivalent circuit diagram of a branchcircuit 2. FIG. 2 is a plan view of the branch circuit 2 from above.FIG. 3 is a plan view of a circuit substrate 4 from above. FIG. 4 is aplan view of the circuit substrate 4 from below. In FIGS. 2 to 4, adirection normal to a main surface of the circuit substrate 4 is definedas an up-down direction. In addition, when the circuit substrate 4 isviewed in plan from above, a direction in which its long edges extend isdefined as a left-right direction and a direction in which its shortedges extend is defined as a front-back direction.

First, a circuit configuration of the branch circuit 2 will be describedwhile referring to FIG. 1. The branch circuit 2 preferably has a circuitconfiguration including an input port P11, output ports P12 and P13, aninput path I1, and output paths O1 and O2. The input port P11 is aterminal though which a high-frequency signal is input. The output portP12 is a terminal through which a high-frequency signal having arelatively high frequency out of the high-frequency signal input fromthe input port P11 is output. The output port P13 is a terminal throughwhich a high-frequency signal having a relatively low frequency out ofthe high-frequency signal input from the input port P11 is output.Therefore, a high-frequency signal is transmitted from the input portP11 to the output ports P12 and P13. Hereafter, a side located in adirection from the output ports P12 and P13 to the input port P11 willbe referred to as an upstream side, and a side located in a directionfrom the input port P11 to the output ports P12 and P13 will be referredto as a downstream side.

An upstream-side end portion of the input path I1 is connected to theinput port P11. The high-frequency signal input from the input port P11is transmitted along the input path I1.

An upstream-side end portion of the output path O1 is connected to adownstream-side end portion of the input path I1. A downstream-side endportion of the output path O1 is connected to the output port P12. Theoutput path O1 includes a matching circuit MC1 and a high pass filterHPF.

The matching circuit MC1 preferably includes a capacitor element C1,resistor elements R1 and R2 and an inductor element L1 and is a circuitconfigured to perform impedance matching. The capacitor element C1, theresistor elements R1 and R2 and the inductor element L1 preferablyinclude lumped-parameter elements (chip components). The capacitorelement C1 and the resistor element R2 are connected in series in theoutput path O1. In addition, the resistor element R1 and the inductorelement L1 are connected in series between a portion where the capacitorelement C1 and the resistor element R2 are connected to each other, andthe ground.

The high pass filter HPF allows a high-frequency signal having afrequency higher than a cutoff frequency f1 to pass therethrough andpreferably includes a capacitor and an inductor. The high pass filterHPF preferably includes a lumped-parameter element (chip component). Thehigh pass filter HPF is connected downstream of the resistor element R2and upstream of the output port P12 in the output path O1.

An upstream-side end portion of the output path O2 is connected to thedownstream-side end portion of the input path I1. A downstream-side endportion of the output path O2 is connected to the output port P13. Theoutput path O2 includes a matching circuit MC2 and a low pass filterLPF.

The matching circuit MC2 preferably includes inductor elements L2 andL3, a resistor element R3 and a capacitor element C2 and is a circuitfor obtaining impedance matching. The inductor elements L2 and L3, theresistor element R3 and the capacitor element C2 preferably includelumped-parameter elements (chip components). The inductor element L2 andthe inductor element L3 are connected in series in the output path O2.In addition, the resistor element R3 and the capacitor element C2 areconnected in series between a portion where the inductor element L2 andthe inductor element L3 are connected, and the ground.

The low pass filter LPF allows a high-frequency signal having afrequency lower than a cutoff frequency f2 to pass therethrough andpreferably includes a capacitor and an inductor. The low pass filter LPFpreferably includes a lumped-parameter element (chip component). The lowpass filter LPF is connected downstream of the inductor element L3 andupstream of the output port P13 in the output path O2.

Next, configurations of the branch circuit 2 and the circuit substrate 4will be described while referring to FIGS. 2 to 4. The branch circuit 2includes the circuit substrate 4, the input port P11, the output portsP12 and P13, the input path I1, and the output paths O1 and O2. In FIGS.2 to 4, the input port P11 and the output ports P12 and P13 are notillustrated.

The circuit substrate 4 illustrated in FIG. 3 includes a substrate body6, signal line conductors 10, 12 a to 12 d and 14 a to 14 d, landportions 16 a to 16 c and 18 a to 18 c and ground conductors G1 and G2.The substrate body 6 preferably is a plate-shaped or substantiallyplate-shaped member having a rectangular or substantially rectangularshape when viewed in plan from above. Hereafter, a main surface of thesubstrate body 6 on the upper side will be referred to as a top surfaceand a main surface of the substrate body 6 on the lower side will bereferred to as a bottom surface.

The ground conductors G1 and G2 are maintained at a ground potential.The ground conductor G1 covers substantially the entire top surface ofthe substrate body 6. The ground conductor G2 covers the entire bottomsurface of the substrate body 6. The ground conductor G1 and the groundconductor G2 are connected to each other by a multiplicity of via holeconductors that penetrate through the substrate body 6 in the up-downdirection.

The signal line conductor 10 is provided on the top surface of thesubstrate body 6 and is included in the input path I1. The groundconductor G1 is not provided around the periphery of the signal lineconductor 10. In other words, the ground conductor G1 is configured sothat there is a gap on the left and right sides of the signal lineconductor 10. In this way, a coplanar waveguide is provided by thesignal line conductor 10 and the ground conductor G1. In addition, theground conductor G2 covers the entire bottom surface of the substratebody 6 and is therefore superposed with the signal line conductor 10when viewed in plan from above. Thus, a coplanar waveguide including theground is provided on the bottom surface side.

The signal line conductor 10 is a line-shaped or substantiallyline-shaped conductor that extends from the center of an edge of thesubstrate body 6 on the front side toward the back side. A back-side endportion of the signal line conductor 10 is positioned in the vicinity ofthe center (diagonal point of intersection) of the substrate body 6. Inaddition, two solder portions are provided in a line on the left andright of the back-side end portion of the signal line conductor 10 sothat a chip component, which will be described later, is able to bemounted. The solder portions are formed preferably by applying solderonto the conductor and in FIG. 3 are illustrated by black shading.

The signal line conductors 12 a to 12 d are provided on the top surfaceof the substrate body 6 and are included in the output path O2. Theground conductor G1 is not provided around the peripheries of the signalline conductors 12 a to 12 d. In other words, the ground conductor G1 isconfigured so that there is a gap around the peripheries of the signalline conductors 12 a to 12 d. In this way, coplanar waveguides aredefined by the signal line conductors 12 a to 12 d and the groundconductor G1. In addition, the ground conductor G2 covers the entirebottom surface of the substrate body 6 and is therefore superposed withthe signal line conductors 12 a to 12 d when viewed in plan from above.Thus, a coplanar waveguide including the ground is provided on thebottom surface side.

The signal line conductor 12 a is positioned behind the back-side endportion of the signal line conductor 10 and is a line-shaped orsubstantially line-shaped conductor that extends in the front-backdirection. Solder portions are provided in three places, that is, on afront-side end portion of the signal line conductor 12 a, on a back-sideend portion of the signal line conductor 12 a and in the center of thesignal line conductor 12 a.

The signal line conductor 12 b is positioned to the right of theback-side end portion of the signal line conductor 12 a and is aline-shaped or substantially line-shaped conductor that extends in theleft-right direction. Solder portions preferably are provided in twoplaces, that is, on a left-side end portion of the signal line conductor12 b and on a right-side end portion of the signal line conductor 12 b.The solder portion provided on the right-side end portion of the signalline conductor 12 b is the land portion 16 a.

The signal line conductor 12 c is positioned to the right of theright-side end portion of the signal line conductor 12 b and is aline-shaped or substantially line-shaped conductor that extends in theleft-right direction. A right-side end portion of the signal lineconductor 12 c is positioned in the center of an edge of the substratebody 6 on the right side. A solder portion, which is the land portion 16c, is provided on a left-side end portion of the signal line conductor12 c.

The signal line conductor 12 d is positioned to the left of the signalline conductor 12 a and is a line-shaped or substantially line-shapedconductor that extends in the front-back direction. Solder portions areprovided in two places, that is, on a front-side end portion of thesignal line conductor 12 d and on a back-side end portion of the signalline conductor 12 d. In addition, a solder portion is provided on theground conductor G1 behind the back-side end portion of the signal lineconductor 12 d.

The land portion 16 b is a solder portion provided between theright-side end portion of the signal line conductor 12 b and theleft-side end portion of the signal line conductor 12 c. The landportion 16 b is formed preferably by applying solder onto the groundconductor G1. The land portions 16 a to 16 c are provided on the topsurface of the substrate body 6 and are included in the output path O2.In addition, the land portions 16 a to 16 c are mount portions on whichthe low pass filter LPF, which will be described later, is to bemounted.

The signal line conductors 14 a to 14 d are provided on the top surfaceof the substrate body 6 and are included in the output path O1. Theground conductor G1 is not provided around the peripheries of the signalline conductors 14 a to 14 d. In other words, the ground conductor G1 isconfigured so that there is a gap around the peripheries of the signalline conductors 14 a to 14 d. In this way, coplanar waveguides aredefined by the signal line conductors 14 a to 14 d and the groundconductor G1. In addition, the ground conductor G2 covers the entirebottom surface of the substrate body 6 and is therefore superposed withthe signal line conductors 14 a to 14 d when viewed in plan from above.Thus, the signal line conductors 14 a to 14 d and the ground conductorG2 define a microstripline structure.

The signal line conductor 14 a is positioned to the left of theback-side end portion of the signal line conductor 10 and is aline-shaped or substantially line-shaped conductor that extends in theleft-right direction. Solder portions preferably are provided in threeplaces, that is, on a right-side end portion of the signal lineconductor 14 a, on a left-side end portion of the signal line conductor14 a and in the center of the signal line conductor 14 a.

The signal line conductor 14 b is positioned behind the left-side endportion of the signal line conductor 14 a and is a line-shaped orsubstantially line-shaped conductor that extends in the front-backdirection. Solder portions preferably are provided in two places, thatis, on a front-side end portion of the signal line conductor 14 b and ona back-side end portion of the signal line conductor 14 b. The solderportion provided on the back-side end portion of the signal lineconductor 14 b is the land portion 18 a.

The signal line conductor 14 c is positioned behind the back-side endportion of the signal line conductor 14 b and is a line-shaped orsubstantially line-shaped conductor that extends in the front-backdirection. A back-side end portion of the signal line conductor 14 c ispositioned in the center of an edge of the substrate body 6 on the backside. A solder portion, which is the land portion 18 c, is provided on afront-side end portion of the signal line conductor 14 c.

The signal line conductor 14 d is positioned in front of the signal lineconductor 14 a and is a line-shaped or substantially line-shapedconductor that extends in the left-right direction. Solder portions areprovided in two places, that is, on a right-side end portion of thesignal line conductor 14 d and on a left-side end portion of the signalline conductor 14 d. In addition, a solder portion is provided on theground conductor G1 to the left of the left-side end portion of thesignal line conductor 14 d.

The land portion 18 b is a solder portion provided between the back-sideend portion of the signal line conductor 14 b and the front-side endportion of the signal line conductor 14 c. The land portion 18 b isformed preferably by applying solder onto the ground conductor G1. Theland portions 18 a to 18 c are provided on the top surface of thesubstrate body 6 and are included in the output path O1. In addition,the land portions 18 a to 18 c are mount portions on which the high passfilter HPF, which will be described later, is to be mounted.

Roughly speaking, the signal line conductors 12 a to 12 d extend fromthe center of the substrate body 6 toward the right and the signal lineconductors 14 a to 14 d extend from the center of the substrate body 6toward the back. Therefore, the signal line conductors 12 a to 12 d andthe signal line conductors 14 a to 14 d have asymmetrical structureswith respect to the signal line conductor 10.

The inductor element L2 is mounted on a right-side solder portionprovided on the back-side end portion of the signal line conductor 10and on a solder portion provided on the front-side end portion of thesignal line conductor 12 a. In other words, an upstream-side end portion(front-side end portion) of the signal line conductor 12 a providedfarthest upstream in a transmission direction of a high-frequency signalamong the signal line conductors 12 a to 12 d is connected to the signalline conductor 10 via the inductor element L2, which is alumped-parameter element.

The inductor element L3 is mounted on the solder portion provided on theback-side end portion of the signal line conductor 12 a and on thesolder portion provided on the left-side end portion of the signal lineconductor 12 b. The resistor element R3 is mounted on the solder portionprovided in the center of the signal line conductor 12 a and on thesolder portion provided on the front side of the signal line conductor12 d. The capacitor element C2 is mounted on the solder portion providedon the back side of the signal line conductor 12 d and on the solderportion provided on the ground conductor G1 behind that solder portion.The low pass filter LPF is mounted on the land portions 16 a to 16 c.

The capacitor element C1 is mounted on a left-side solder portionprovided on the back-side end portion of the signal line conductor 10and on a solder portion provided on the right-side end portion of thesignal line conductor 14 a. In other words, an upstream-side end portionof the signal line conductor 14 a (right-side end portion) providedfarthest upstream in a transmission direction of a high-frequency signalamong the signal line conductors 14 a to 14 d is connected to the signalline conductor 10 via the capacitor element C1, which is alumped-parameter element.

The resistor element R2 is mounted on the solder portion provided on theleft-side end portion of the signal line conductor 14 a and on a solderportion provided on the front-side end portion of the signal lineconductor 14 b. The resistor element R1 is mounted on the solder portionprovided in the center of the signal line conductor 14 a and on thesolder portion provided on the right side of the signal line conductor14 d. The inductor element L1 is mounted on the solder portion providedon the left side of the signal line conductor 14 d and on the solderportion provided on the ground conductor G1 to the left of that solderportion. The high pass filter HPF is mounted on the land portions 18 ato 18 c.

In the above-described branch circuit 2, the capacitor element C2 isarranged closer to the high pass filter HPF than to the low pass filterLPF and is connected to the ground conductor G1. Accordingly, thematching circuit MC2 is connected to the ground conductor G1 at a pointcloser to the high pass filter HPF than to the low pass filter LPF. Thatis, the matching circuit MC2 is connected to the ground conductor G1 ata position spaced apart from the low pass filter LPF.

With the thus-configured branch circuit 2 and circuit substrate 4, lossgenerated in the output path O1 is significantly reduced or prevented.In more detail, an upstream-side end portion of the signal lineconductor 14 a provided farthest upstream in a transmission direction ofa high-frequency signal among the signal line conductors 14 a to 14 d isconnected to the signal line conductor 10 via the capacitor element C1,which is a lumped-parameter element. That is, the output path O1 isconnected to the signal line conductor 10 in a form not including anunnecessary line conductor, that is, not via a signal line conductor butvia the capacitor element C1. Thus, a signal line conductor, which is aninductor component, does not exist between the matching circuit MC1 andthe signal line conductor 10. Therefore, disruption of impedancematching between the input port P11 and the output port P12 is preventedand loss generated in the output path O1 is significantly reduced orprevented.

In addition, with the thus-configured branch circuit 2 and circuitsubstrate 4, loss generated in the output path O2 is significantlyreduced or prevented. In more detail, an upstream-side end portion ofthe signal line conductor 12 a provided farthest upstream in atransmission direction of a high-frequency signal among the signal lineconductors 12 a to 12 d is connected to the signal line conductor 10 viathe inductor element L2, which is a lumped-parameter element. That is,the output path O2 is connected to the signal line conductor 10 not viaa signal line conductor but via the inductor element L2. Thus, a signalline conductor, which is an inductor component, does not exist betweenthe matching circuit MC2 and the signal line conductor 10. Therefore,disruption of impedance matching between the input port P11 and theoutput port P13 is prevented and loss generated in the output path O2 issignificantly reduced or prevented.

In addition, with the branch circuit 2 and the circuit substrate 4, thematching circuit MC2 is connected to the ground conductor G1 at a pointcloser to the high pass filter HPF than to the low pass filter LPF.Thus, as is clear from experimental results and simulation results to bedescribed later, two attenuation poles are generated in a frequency bandhigher than the cutoff frequency f2 of the low pass filter LPF in ahigh-frequency signal output from the output port P13. Therefore, theband of the output path O2 is widened.

In addition, with the branch circuit 2 and the circuit substrate 4, thesignal line conductors 12 a to 12 d and the signal line conductors 14 ato 14 d have asymmetrical structures with respect to the signal lineconductor 10. Thus, the degree of freedom as regards the layout of thearrangement of the signal line conductors 12 a to 12 d and 14 a to 14 dis increased.

In addition, coplanar waveguides are defined by the signal lineconductors 10, 12 a to 12 d and 14 a to 14 d and the ground conductorG1. Furthermore, the signal line conductors 10, 12 a to 12 d and 14 a to14 d and the ground conductor G2 define coplanar waveguides including aground on the bottom surface. Thus, the characteristic impedances of thesignal line conductors 10, 12 a to 12 d and 14 a to 14 d preferably aremade to match predetermined values (for example, about 50Ω and about75Ω).

The inventors of various preferred embodiments of the presentapplication carried out the experiments described below in order to makethe broadening of a frequency range that is attenuated in the outputpath O2 in the branch circuit 2 and the circuit substrate 4 clearer.FIG. 5 is a plan view of a branch circuit 2 a according to amodification to one of the preferred embodiments of the presentinvention described above. In the branch circuit 2 a illustrated in FIG.5, the capacitor element C2 is arranged closer to the low pass filterLPF than to the high pass filter HPF and is connected to the ground G1.Accordingly, the matching circuit MC2 is connected to the groundconductor G1 at a point closer to the low pass filter LPF than to thehigh pass filter HPF.

The inventors of various preferred embodiments of the presentapplication fabricated a first sample of the branch circuit 2illustrated in FIG. 2 and a second sample of the branch circuit 2 aillustrated in FIG. 5. An S parameter was measured for the first sampleand the second sample. FIG. 6 is a graph illustrating a transmissioncharacteristic S21 for the first sample. FIG. 7 is a graph illustratinga transmission characteristic S21 for the second sample. Thetransmission characteristic S21 is a parameter representing the ratio ofthe strength of a high-frequency signal output from the output port P13to the strength of a signal input from the input port P11.

The cutoff frequency f2 of the low pass filter LPF of the first sampleand the second sample is around 1.1 GHz. According to FIG. 7, noattenuation poles are generated in the second sample around 2.5 GHz,which is a frequency higher than the cutoff frequency f2. On the otherhand, according to FIG. 6, two attenuation poles are generated in thefirst sample around 2.5 GHz, which is a frequency higher than the cutofffrequency f2. Thus, in the first sample, it is clear that largeattenuation is obtained in a frequency band higher than the cutofffrequency f2 and that broadening of a frequency range that is attenuatedis achieved.

Next, the inventors of various preferred embodiments of the presentapplication performed the computer simulation described below in orderto make the cause of the broadening of the frequency range that isattenuated in the branch circuit 2 and the circuit substrate 4 clearer.FIG. 8 is an equivalent circuit diagram of a first model correspondingto the first sample. FIG. 9 is an equivalent circuit diagram of a secondmodel corresponding to the second sample.

The first model is different from the second model in that an inductorcomponent is provided between the low pass filter LPF and the ground.Hereafter, the reason for providing the inductor component between thelow pass filter LPF and the ground in the first model will be described.

In the branch circuit 2 a (corresponding to the second sample and secondmodel) illustrated in FIG. 5, the solder portion with which thecapacitor element C2 and the ground conductor G1 are connected to eachother and the solder portion (land 16 b) with which the low pass filterLPF and the ground conductor G1 are connected to each other arerelatively close to each other. Consequently, almost no inductorcomponent is generated along a current path from the solder portion withwhich the capacitor element C2 and the ground conductor G1 are connectedto each other and the solder portion with which the low pass filter LPFand the ground conductor G1 are connected to each other. Therefore, inthe second model illustrated in FIG. 9, an inductor component is notprovided between the low pass filter LPF and the ground.

On the other hand, in the branch circuit 2 (corresponding to the firstsample and first model) illustrated in FIG. 2, the solder portion withwhich the capacitor element C2 and the ground conductor G1 are connectedto each other and the solder portion with which the low pass filter LPFand the ground conductor G1 are connected to each other (land 16 b) arerelatively far apart. Consequently, an inductor component is generatedalong the current path from the solder portion with which the capacitorelement C2 and the ground conductor G1 are connected to each other tothe solder portion with which the low pass filter LPF and the groundconductor G1 are connected to each other. Therefore, in the first model,an inductor component is provided between the low pass filter LPF andthe ground.

An S parameter is calculated in the above-described first model andsecond model. FIG. 10 is a graph illustrating a transmissioncharacteristic S21 of the first model and the second model.

According to FIG. 10, one attenuation pole is generated in the secondmodel around 2.5 GHz, which is a frequency higher than the cutofffrequency f2. On the other hand, according to FIG. 10, two attenuationpoles are generated in the first model around 2.5 GHz, which is afrequency higher than the cutoff frequency f2, similarly to as in thefirst sample. Therefore, it is clear that two attenuation poles aregenerated because the solder portion with which the capacitor element C2and the ground conductor G1 are connected to each other and the solderportion with which the low pass filter LPF and the ground conductor G1are connected to each other (land portion 16 b) are spaced relativelyfar apart from each other as in the first model. Thus, it is clear thatlarge attenuation is obtained in a frequency band higher than the cutofffrequency f2 and that broadening of a frequency range that is attenuatedis achieved.

Next, the inventors of various preferred embodiments of the presentapplication performed the computer simulation described below in orderto make is clearer why loss generated in the output path O1 in thebranch circuit 2 and the circuit substrate 4 is reduced. FIG. 11 is anequivalent circuit diagram of a model used in a computer simulation.

In the model illustrated in FIG. 11, wiring lines Line1 and Line2 wererespectively arranged upstream and downstream of the capacitor elementC1 and wiring lines Line3 and Line4 were respectively arranged upstreamand downstream of the inductor element L2. Wiring lines Line1 to Line4preferably include signal line conductors. The lengths of the wiringlines Line1 to Line4 were changed and then the S parameter wascalculated for each model.

Third Model: length of wiring line Line1 2 mm, length of wiring lineLine2 2 mm, length of wiring line Line3 2 mm, length of wiring lineLine4 2 mm

Fourth Model: length of wiring line Line1 0 mm, length of wiring lineLine2 4 mm, length of wiring line Line3 2 mm, length of wiring lineLine4 around 2 mm

Fifth Model: length of wiring line Line1 0 mm, length of wiring lineLine2 4 mm, length of wiring line Line3 0 mm, length of wiring lineLine4 4 mm

In addition, hereafter, conditions common to the third to fifth modelswill be described.

Capacitance of capacitor element C1: 1.8 pF

Inductance of inductor element L1: 10 nH

Inductance of inductor element L2: 15 nH

Capacitance of capacitor element C2: 3.0 pF

The third model corresponds to a comparative example. In the thirdmodel, signal line conductors (wiring lines Line1 and Line3) existbetween the input path I1 and the matching circuit MC1 of the outputpath O1 and between the input path I1 and the matching circuit MC2 ofthe output path O2.

The fourth model corresponds to a preferred embodiment of the presentinvention. In the fourth model, a signal line conductor (wiring lineLine1) does not exist between the input path I1 and the matching circuitMC1 of the output path O1 and a signal line conductor (wiring lineLine3) does exist between the input path I1 and the matching circuit MC2of the output path O2.

In other words, in the fourth model, the signal line conductor thatexists between the input path I1 and the matching circuit MC1 of theoutput path O1 in the third model (wiring line Line1) is shifted to thesignal line conductor downstream of the capacitor element C1 (wiringline Line2).

The fifth model corresponds to a preferred embodiment of the presentinvention. In the fifth model, signal line conductors (wiring linesLine1 and Line3) do not exist between the input path I1 and the matchingcircuit MC1 of the output path O1 and between the input path I1 and thematching circuit MC2 of the output path O2. In other words, in the fifthmodel, the signal line conductor that exists between the input path I1and the matching circuit MC1 of the output path O1 in the third model(wiring line Line1) is shifted to the signal line conductor downstreamof the capacitor element C1 (wiring line Line2). In addition, in thefifth model, the signal line conductor that exists between the inputpath I1 and the matching circuit MC2 of the output path O2 in the thirdmodel (wiring line Line3) is shifted to the signal line conductordownstream of the inductor element L2 (wiring line Line4).

The inventors of preferred embodiments of the present applicationcalculated a reflection characteristic S33 using the third to fifthmodels. The reflection characteristic S33 is a parameter representingthe ratio of the strength of a high-frequency signal output from theoutput port P12 to the strength of a signal input from the output portP12. FIG. 12 is a graph illustrating the reflection characteristic S33in the third model. FIG. 13 is a graph illustrating the reflectioncharacteristic S33 in the fourth model. FIG. 14 is a graph illustratingthe reflection characteristic S33 in the fifth model.

Comparing FIG. 12 and FIG. 13, it is clear that the decrease in thereflection characteristic S33 around 1.1 GHz is larger in the fourthmodel than in the third model. Thus, the reflection characteristic S33is improved by removing the signal line conductor from between the inputpath I1 and the matching circuit MC1 of the output path O1. That is, itis clear that loss generated in the output path O1 is improved.

In addition, comparing FIG. 13 and FIG. 14, it is clear that thedecrease in the reflection characteristic S33 around 1.1 GHz is the samein the fourth model and the fifth model. Thus, it is clear that thecause of the large decrease in the reflection characteristic S33 around1.1 GHz was not removal of a signal line conductor from between theinput path I1 and the matching circuit MC2 of the output path O2, butrather removal of a signal line conductor from between the input path I1and the matching circuit MC1 of the output path O1.

A circuit substrate and a branch circuit according to a modification ofa preferred embodiment of the present invention will be described whilereferring to the drawings. FIG. 15 is an equivalent circuit diagram of abranch circuit 2 b. FIG. 16 is a plan view of the branch circuit 2 bfrom above. FIG. 17 is a plan view of a circuit substrate 4 b fromabove. FIG. 18 is a plan view of the circuit substrate 4 b from below.In FIGS. 15 to 18, a direction normal to a main surface of the circuitsubstrate 4 b is defined as an up-down direction.

First, a circuit configuration of the branch circuit 2 b will bedescribed while referring to FIG. 15. The equivalent circuit of thebranch circuit 2 b is different from the equivalent circuit of thebranch circuit 2 in that the resistor elements R2 and R3 and theinductor element L3 are not provided. The rest of the configuration ofthe equivalent circuit of the branch circuit 2 b is preferably the sameas that of the equivalent circuit of the branch circuit 2 and thereforedescription thereof will be omitted.

Next, configurations of the branch circuit 2 b and the circuit substrate4 b will be described while referring to FIGS. 16 to 18. The branchcircuit 2 b includes the circuit substrate 4 b, an input port P11,output ports P12 and P13, an input path I1, and output paths O1 and O2.In FIGS. 16 to 18, the input port P11 and the output ports P12 and P13are not illustrated.

The circuit substrate 4 b includes a substrate body 6, signal lineconductors 10, 12 e, 12 f and 14 e to 14 g, land portions 16 a to 16 cand 18 a to 18 c, and ground conductors G1 and G2. The substrate body 6is a plate-shaped or substantially plate-shaped member having arectangular or substantially rectangular shape when viewed in plan fromabove. Hereafter, a main surface of the substrate body 6 on the upperside will be referred to as a top surface and a main surface of thesubstrate body 6 on the lower side will be referred to as a bottomsurface.

The ground conductors G1 and G2 of the circuit substrate 4 b arepreferably the same as the ground conductors G1 and G2 of the circuitsubstrate 4 and therefore description thereof is omitted.

The signal line conductor 10 of the circuit substrate 4 b is preferablythe same as the signal line conductor 10 of the circuit substrate 4 andtherefore detailed description thereof is omitted. In addition, twosolder portions are provided in a line on the left and right of aback-side end portion of the signal line conductor 10 so that a chipcomponent, which will be described later, is able to be mounted. Thesolder portions are formed preferably by applying solder onto theconductor and in FIG. 17 are illustrated by black shading.

The signal line conductors 12 e and 12 f are provided on the top surfaceof the substrate body 6 and are included in the output path O2. Theground conductor G1 is not provided around the peripheries of the signalline conductors 12 e and 12 f. In this way, coplanar waveguides aredefined by the signal line conductors 12 e and 12 f and the groundconductor G1. In addition, the ground conductor G2 covers the entirebottom surface of the substrate body 6 and is therefore superposed withthe signal line conductors 12 e and 12 f when viewed in plan from above.Thus, the signal line conductors 12 e and 12 f and the ground conductorG2 form coplanar waveguides including a ground on the bottom surface.

The signal line conductor 12 e is positioned to the right of theback-side end portion of the signal line conductor 10 and is aline-shaped or substantially line-shaped conductor that extends in theleft-right direction. Solder portions preferably are provided in threeplaces, that is, on a left-side end portion of the signal line conductor12 e, on a right-side end portion of the signal line conductor 12 e andin the center of the signal line conductor 12 e. The solder portionprovided on the right-side end portion of the signal line conductor 12 eis the land portion 16 a.

The signal line conductor 12 f is positioned to the right of aright-side end portion of the signal line conductor 12 e and is aline-shaped or substantially line-shaped conductor that extends in theleft-right direction. A right-side end portion of the signal lineconductor 12 f is positioned at an edge of the substrate body 6 on theright side. A solder portion, which is the land portion 16 c, isprovided on a left-side end portion of the signal line conductor 12 f.

The land portion 16 b is a solder portion provided between theright-side end portion of the signal line conductor 12 e and theleft-side end portion of the signal line conductor 12 f. The landportion 16 b is formed preferably by applying solder onto the groundconductor G1. The land portions 16 a to 16 c are provided on the topsurface of the substrate body 6 and are included in the output path O2.In addition, the land portions 16 a to 16 c are land portions on whichthe low pass filter LPF, which will be described later, is to bemounted.

The signal line conductors 14 e to 14 g are provided on the top surfaceof the substrate body 6 and are included in the output path O1. Theground conductor G1 is not provided around the peripheries of the signalline conductors 14 e to 14 g. In this way, coplanar waveguides aredefined by the signal line conductors 14 e to 14 g and the groundconductor G1. In addition, the ground conductor G2 covers the entirebottom surface of the substrate body 6 and is therefore superposed withthe signal line conductors 14 e to 14 g when viewed in plan from above.Thus, the signal line conductors 14 e to 14 g and the ground conductorG2 define coplanar waveguides including a ground on the bottom surface.

The signal line conductor 14 e is positioned to the left of theback-side end portion of the signal line conductor 10 and is aline-shaped or substantially line-shaped conductor that extends in theleft-right direction. Solder portions are preferably provided in threeplaces, that is, on a right-side end portion of the signal lineconductor 14 e, on a left-side end portion of the signal line conductor14 e and in the center of the signal line conductor 14 e. The solderportion provided on the left-side end portion of the signal lineconductor 14 e is the land portion 18 a.

The signal line conductor 14 f is positioned to the left of a left-sideend portion of the signal line conductor 14 e and is a line-shaped orsubstantially line-shaped conductor that extends in the left-rightdirection. A left-side end portion of the signal line conductor 14 f ispositioned at an edge of the substrate body 6 on the left side. A solderportion, which is the land portion 18 c, is provided on a right-side endportion of the signal line conductor 14 f.

The signal line conductor 14 g is positioned behind the signal lineconductor 14 e and is a line-shaped or substantially line-shapedconductor that extends in the front-back direction. Solder portions arepreferably provided in two places, that is, on a front-side end portionof the signal line conductor 14 g and on a back-side end portion of thesignal line conductor 14 g. In addition, a solder portion is provided onthe ground conductor G1 behind the back-side end portion of the signalline conductor 14 g.

The land portion 18 b is a solder portion provided between the left-sideend portion of the signal line conductor 14 e and the right-side endportion of the signal line conductor 14 f. The land portion 18 b isformed by applying solder onto the ground conductor G1.

As described above, roughly speaking, the signal line conductors 12 eand 12 f extend toward the right from the center of the substrate body6. Roughly speaking, the signal line conductors 14 e to 14 g extendtoward the left from the center of the substrate body 6. Thus, thesignal line conductors 12 e and 12 f and the signal line conductors 14 eto 14 g have line-symmetrical or substantially line-symmetricalstructures with respect to the signal line conductor 10.

The inductor element L2 illustrated in FIG. 16 is mounted on aright-side solder portion provided on the back-side end portion of thesignal line conductor 10 and on a solder portion provided on theleft-side end portion of the signal line conductor 12 e. In other words,an upstream-side end portion (left-side end portion) of the signal lineconductor 12 e provided farthest upstream in a transmission direction ofa high-frequency signal among the signal line conductors 12 e and 12 fis connected to the signal line conductor 10 via the inductor elementL2, which is a lumped-parameter element. That is, it can be connectedwithout using an unnecessary line conductor.

The capacitor element C2 illustrated in FIG. 15 is mounted on the solderportion provided in the center of the signal line conductor 12 e and onthe solder portion provided on the ground conductor G1 behind thatsolder portion. The low pass filter LPF is mounted on the land portions16 a to 16 c.

The capacitor element C1 is mounted on the left-side solder portionprovided on the back-side end portion of the signal line conductor 10and on the solder portion provided on the right-side end portion of thesignal line conductor 14 e. In other words, an upstream-side end portionof the signal line conductor 14 e (right-side end portion) providedfarthest upstream in a transmission direction of a high-frequency signalamong the signal line conductors 14 e to 14 g is connected to the signalline conductor 10 via the capacitor element C1, which is alumped-parameter element. That is, it is connected without using anunnecessary line conductor.

The resistor element R1 is mounted on the solder portion provided in thecenter of the signal line conductor 14 e and on the solder portionprovided on the front-side end portion of the signal line conductor 14g. The inductor element L1 is mounted on the solder portion provided onthe back-side end portion of the signal line conductor 14 g and on thesolder portion provided on the ground conductor G1 behind that solderportion. The high pass filter HPF is mounted on the land portions 18 ato 18 c.

The thus-configured branch circuit 2 b and circuit substrate 4 b arealso able to exhibit the same effect as the branch circuit 2 and thecircuit substrate 4.

Other Preferred Embodiments

Branch circuits and circuit substrates according to the presentinvention are not limited to the branch circuits 2, 2 a and 2 b and thecircuit substrates 4, 4 a and 4 b and may be modified within the spiritof the present invention.

In addition, the configurations of the branch circuits 2, 2 a and 2 band the circuit substrates 4, 4 a and 4 b may be appropriately combined.

Furthermore, the branch circuits 2, 2 a and 2 b and the circuitsubstrates 4, 4 a and 4 b include two output paths O1 and O2, but mayinstead include three or more output paths, for example.

The ground conductor G2 is provided on the bottom surface of thesubstrate body 6, but may instead be provided inside the substrate body6.

In addition, in the branch circuits 2 and 2 a and the circuit substrates4 and 4 a, an upstream-side end portion of the signal line conductor 14a provided farthest upstream in a transmission direction of ahigh-frequency signal among the signal line conductors 14 a to 14 d neednot be connected to the signal line conductor 10 via the capacitorelement C1, which is a lumped-parameter element. That is, the signalline conductor 14 a may be directly connected to the signal lineconductor 10. Similarly, in the branch circuit 2 b and the circuitsubstrate 4 b, an upstream-side end portion of the signal line conductor14 e provided farthest upstream in a transmission direction of ahigh-frequency signal among the signal line conductors 14 e to 14 g neednot be connected to the signal line conductor 10 via the capacitorelement C1, which is a lumped-parameter element. That is, the signalline conductor 14 e may be directly connected to the signal lineconductor 10.

In addition, the capacitor element C1, which connects the upstream-sideend portion of the signal line conductor 14 a provided farthest upstreamin a transmission direction of a high-frequency signal among the signalline conductors 14 a to 14 d and the signal line conductor 10, maydefine a portion of the high pass filter HPF instead of defining aportion of the matching circuit MC1.

Furthermore, the high pass filter HPF and the low pass filter LPF may beeach defined by a plurality of chip components rather just one chipcomponent.

In the branch circuits 2, 2 a and 2 b and the circuit substrates 4, 4 aand 4 b, it is preferable that the winding axis of the inductor elementL1 and the winding axis of an inductor included in the high pass filterHPF be perpendicular or substantially perpendicular to each other. Forexample, it is preferable that the winding axis of the inductor elementL1 extend in the front-back direction and that the winding axis of aninductor included in the high pass filter HPF extend in the left-rightdirection. In this way, magnetic field coupling between the inductorelement L1 and the inductor included in the high pass filter HPF issignificantly reduced or prevented.

In the branch circuits 2, 2 a and 2 b and the circuit substrates 4, 4 aand 4 b, the signal line conductors 10, 12 a to 12 f and 14 a to 14 gpreferably include conductor layers provided on the top surface of thesubstrate body 6, but they may include conductor layers provided insidethe substrate body 6.

In the branch circuits 2 and 2 a and the circuit substrates 4 and 4 a,the capacitor element C1 may be connected to a portion of the signalline conductor 14 a other than the upstream-side portion. Similarly, inthe branch circuit 2 b and the circuit substrate 4 b, the capacitorelement C1 may be connected to a portion of the signal line conductor 14e other than the upstream-side portion.

In addition, in the branch circuits 2 and 2 a and the circuit substrates4 and 4 a, the inductor element L2 may be connected to a portion of thesignal line conductor 12 a other than the upstream-side portion.Similarly, in the branch circuit 2 b and the circuit substrate 4 b, theinductor element L2 may be connected to a portion of the signal lineconductor 12 e other than the upstream-side portion.

In addition, in the branch circuits 2 and 2 a and the circuit substrates4 and 4 a, the signal line conductors 10, 12 a to 12 f and 14 a to 14 gmay define a microstripline structure.

Furthermore, in the branch circuits 2 and 2 a and the circuit substrates4 and 4 a, other passive elements such as capacitor elements or inductorelements may be provided instead of the resistor elements R1 to R3.

In addition, in the branch circuits 2 and 2 a and the circuit substrates4 and 4 a, the positional relationship between the output path O1 andthe output path O2 may be reversed.

Preferred embodiments of the present invention are applicable to branchcircuits and circuit substrates and are particularly excellent in thatgeneration of loss is significantly reduced or prevented.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A circuit substrate for use in a branch circuitthat includes an input path, a first output path including a high passfilter and connected to the input path, and a second output pathincluding a low pass filter and connected to the input path, the circuitsubstrate comprising: a substrate body; an input signal line conductorwith which the substrate body is provided and which is included in theinput path; a first mounting portion provided on a main surface of thesubstrate body, included in the first output path and on which the highpass filter is mounted; at least one first output signal line conductorwith which the substrate body is provided and which is included in thefirst output path; and a second mounting portion provided on the mainsurface of the substrate body, included in the second output path and onwhich the low pass filter is mounted; wherein the first output signalline conductor provided farthest upstream in a propagation direction ofa signal is connected to the input signal line conductor via a firstlumped-parameter element mounted on the main surface of the substratebody.
 2. The circuit substrate according to claim 1, wherein the firstoutput path includes a first matching circuit including the firstlumped-parameter element.
 3. The circuit substrate according to claim 2,wherein the first matching circuit includes an inductor element, whereinthe high pass filter includes an inductor; and a winding axis of theinductor element and a winding axis of an inductor are perpendicular orsubstantially perpendicular to each other.
 4. The circuit substrateaccording to claim 1, further comprising: at least one second outputsignal line conductor with which the substrate body is provided andwhich is included in the second output path; wherein the second outputsignal line conductor provided farthest upstream in a propagationdirection of a signal is connected to the input signal line conductorvia a second lumped-parameter element mounted on the main surface of thesubstrate body.
 5. The circuit substrate according to claim 4, whereinthe second output path includes a second matching circuit including thesecond lumped-parameter element.
 6. The circuit substrate according toclaim 5, further comprising: a first ground conductor provided on themain surface of the substrate body and maintained at a ground potential;and the second matching circuit connected to the first ground conductorat a point closer to the high pass filter than to the low pass filter.7. The circuit substrate according to claim 4, wherein the first outputsignal line conductor and the second output signal line conductor haveasymmetrical structures with respect to the input signal line conductor.8. The circuit substrate according to claim 1, further comprising afirst ground conductor provided on the main surface of the substratebody and maintained at a ground potential, the input signal lineconductor defining a coplanar waveguide together with the first groundconductor.
 9. The circuit substrate according to claim 1, furthercomprising a second ground conductor with which the substrate body isprovided and which is superposed with the input signal line conductorwhen viewed in plan from a direction normal to the main surface of thesubstrate body.
 10. A branch circuit comprising: the circuit substrateaccording to claim 1; the high pass filter mounted on the first mountingportion; and the low pass filter mounted on the second mounting portion.11. The circuit substrate according to claim 2, further comprising: atleast one second output signal line conductor with which the substratebody is provided and which is included in the second output path;wherein the second output signal line conductor located farthestupstream in a propagation direction of a signal is connected to theinput signal line conductor via a second lumped-parameter elementmounted on the main surface of the substrate body.
 12. The circuitsubstrate according to claim 3, further comprising: at least one secondoutput signal line conductor with which the substrate body is providedand which is included in the second output path; wherein the secondoutput signal line conductor located farthest upstream in a propagationdirection of a signal is connected to the input signal line conductorvia a second lumped-parameter element mounted on the main surface of thesubstrate body.
 13. The circuit substrate according to claim 5, whereinthe first output signal line conductor and the at least one secondoutput signal line conductor have asymmetrical structures with respectto the input signal line conductor.
 14. The circuit substrate accordingto claim 6, wherein the first output signal line conductor and the atleast one second output signal line conductor have asymmetricalstructures with respect to the input signal line conductor.
 15. Thecircuit substrate according to claim 2, further comprising a groundconductor provided on the main surface of the substrate body andmaintained at a ground potential, the input signal line conductordefining a coplanar waveguide together with the ground conductor. 16.The circuit substrate according to claim 3, further comprising a groundconductor provided on the main surface of the substrate body andmaintained at a ground potential, the input signal line conductordefining a coplanar waveguide together with the ground conductor. 17.The circuit substrate according to claim 4, further comprising a groundconductor provided on the main surface of the substrate body andmaintained at a ground potential, the input signal line conductordefining a coplanar waveguide together with the ground conductor. 18.The circuit substrate according to claim 5, further comprising a groundconductor provided on the main surface of the substrate body andmaintained at a ground potential, the input signal line conductordefining a coplanar waveguide together with the ground conductor. 19.The circuit substrate according to claim 6, further comprising a groundconductor provided on the main surface of the substrate body andmaintained at a ground potential, the input signal line conductordefining a coplanar waveguide together with the ground conductor. 20.The circuit substrate according to claim 7, further comprising a groundconductor provided on the main surface of the substrate body andmaintained at a ground potential, the input signal line conductordefining a coplanar waveguide together with the ground conductor.